Method for regulating the speed of a motor vehicle

ABSTRACT

In a method for controlling the speed of a motor vehicle, an acceleration-demand signal is generated which represents a positive or negative setpoint acceleration of the vehicle, and either a control command is output to the engine or a control command is output to braking system of the vehicle as a function of this signal, wherein a signal for preloading the braking system is output when the acceleration-demand signal falls below a threshold value which lies above a value at which the braking system is activated.

FIELD OF THE INVENTION

The present invention relates to a method for controlling the speed of amotor vehicle, in which an acceleration-demand signal is generated whichrepresents a positive or negative setpoint acceleration of the vehicle,and either a control command is output to the engine or a controlcommand is output to the braking system of the vehicle as a function ofthis signal.

BACKGROUND INFORMATION

A method of this type is discussed in the publication “Adaptive CruiseControl System—Aspects and Development Trends” by Winner, Witte, Uhlerand Lichtenberg, Robert Bosch GmbH, in the SAE Technical Paper Series961010, International Congress & Exposition, Detroit, Feb. 26-29, 1996.The control system discussed in this publication, also known as an ACCsystem (adaptive cruise control), is based on a distance sensor, e.g. aradar sensor having multiple-target capability, which is mounted on thefront side of the vehicle in order to measure distances to and relativespeeds of preceding vehicles. As a function of the measurement data ofthis radar system, the speed of a driver's vehicle is then controlled insuch a manner that a predefined distance, which the driver is able todetermine in the form of a so-called setpoint time gap, is maintained tothe immediately preceding vehicle. If there is no preceding vehicle inthe locating range of the radar, control is carried out to a desiredspeed set by the driver.

For example, normally this system intervenes, via a throttle valve, inthe drive system of the vehicle so that the vehicle speed is regulatedvia the drive torque of the engine. However, when, for example, ondownhill grades, or when, necessitated by distance, a strongerdeceleration of the vehicle may be necessary, the drag torque of theengine is not adequate to bring about a sufficient deceleration of thevehicle, then an intervention in the braking system of the vehicle iscarried out.

To permit a stable control and to avoid adversely influencing thecomfort or driving safety, the control system should react with theshortest possible delay time to the control commands output to the drivesystem or braking system. However, upon activation of the vehicle brake,a certain delay in response results due to the fact that the wheel brakecylinders and the remaining components of the hydraulic braking systemhave a certain dead volume which may need to first of all be filled withbrake fluid before the brakes at the individual wheels of the vehicleactually become effective.

German Published Patent Application No. 196 15 294 relates to a devicefor controlling the braking force at the wheels of a vehicle, in whichprior in time to the actual pressure buildup necessary for adjusting therequired braking torque, a small braking pressure in the form of atime-restricted fill pulse is fed into the braking system. Here, thefill pulse is generated individually for the wheel brakes of theindividual wheels as a function of movement variables of the vehicle,particularly as a function of a system deviation of the lateralacceleration of the vehicle and its time derivation. German PublishedPatent Application No. 34 23 063 relates to a traction control systemfor vehicles, in which a small braking pressure is fed as a function ofthe change in the throttle valve position, as a function of the vehiclespeed or as a function of slip thresholds which lie below the responsethreshold for the actual traction control.

SUMMARY

It is an object of the present invention to provide a method forcontrolling speed such that the response time to a control commandoutput to the braking system is shortened.

In an exemplary embodiment of the present invention, a signal forpreloading the braking system is output when the acceleration-demandsignal falls below a threshold value which is above a value at which thebraking system is activated.

To be understood here generally by “preloading” of the braking system isa measure which shifts the braking system into a state in which it isable to react more quickly to a braking command without a substantialbraking action already occurring. For example, this preloading may occurin that a certain braking-pressure buildup takes place in thewheel-brake cylinders and/or in the components of the hydraulic systemwhich are arranged directly in front of them, so that the brake shoesare already approaching or even already touching the brake disk or brakedrum of the wheel brake without, however, a significant frictional forcealready being exerted. Alternatively or additionally, the preloading mayalso be produced by making a certain admission pressure available in apressure accumulator or in a brake booster, which allows the dead volumeto fill up faster upon actual operation of the brake.

Since, according to an exemplary embodiment of the present invention,the signal for preloading the braking system, also known in thefollowing as “fill signal”, is not first generated in the braking systemitself, but rather is already generated within the framework of thespeed control, faster buildup of braking pressure may also be madeuseful for the speed control.

When, in the scope of the speed control, an intervention into thebraking system of the vehicle may be necessary for whatever reason, thismay announce itself in that the acceleration-demand signal becomesnegative and approaches a value at which the deceleration attainable bythe tractive resistance and the drag torque of the engine is no longersufficient, and therefore it may be necessary to switch over to brakingoperation. By comparing the acceleration-demand signal to a suitablethreshold value, it is therefore possible to recognize an imminentswitchover to braking operation early and to preventatively preload thebraking system, so that the braking system reacts more quickly in theevent of actual activation. The shortening of the control delay therebyachieved makes it possible to better suppress unwanted controloscillations. In the case of a distance control, this means at the sametime that falling below the setpoint distance is minimized, so that notonly is comfort improved, but driving safety is increased as well.

Since according to an exemplary embodiment of the present invention, adrop of the acceleration-demand signal below a threshold value isutilized as a criterion for the output of the fill signal, the use ofthe method is independent of which control goals are being pursued withthe speed control and which control algorithms are being used in detail.The method of an exemplary embodiment of the present invention istherefore applicable for a large range of different control concepts,and permits high flexibility with respect to later modifications orexpansions of the control system.

The criterion for the actual activation of the braking system maypossibly be defined in that the acceleration-demand signal falls below atriggering threshold which, on its part, may be varied dynamically as afunction of the driving situation. The threshold value for thepreloading of the braking system may then lie by a fixed differentialamount above this triggering threshold. In practice, a lower limit isdefined for the change rate of the acceleration-demand signal over time,this limit ensuring that the comfort and the feeling of safety for thevehicle passengers are not impaired by decelerations applied in a jerkymanner. On the basis of this limit and on the basis of the fillingtime—determined by the design of the braking system—which may benecessary for preloading the braking system, it is possible tocontinually select the differential amount between the threshold valuefor the preloading of the braking system and the actual triggeringthreshold so that the preloading process is concluded before the brakeis actually activated.

The decisive quantity for determining the triggering threshold may bethe maximum vehicle deceleration, i.e. the minimum (negative)acceleration a_(min) which may be produced by the drag torque of theengine (with the throttle valve closed to the maximum). First andforemost, this minimum acceleration a_(min) is a function of thetransmission step and the operating conditions of the engine, and may becalculated from variables which are made available by the enginemanagement system and other control elements or sensors of the vehiclevia a data bus. For refinement, further criteria may also be utilizedfor determining a_(min), particularly the rise or the gradient of theroadway and, optionally, the payload of the vehicle. The relevantvariables may be measured directly or may be derived indirectly from thecorrelation between the instantaneous output torque of the engine andthe measured vehicle acceleration. The output torque of the engine mayeither be measured directly or be ascertained from the information aboutthe state of the throttle valve, the ignition system, the injectionsystem, etc., available on the data bus.

The fill signal may be canceled again after a specific time span haselapsed. If the acceleration-demand signal remains over a longer time inthe interval between the threshold value for the preloading and thetriggering threshold, because the driving conditions have stabilized inthis area, the brake is thus prevented from remaining pre-loaded for anunnecessarily long time. If the brake is already actually activatedbefore this time span has elapsed, then the fill signal is already resetupon actuation of the brake. In the same manner, the fill signal is alsoreset when the brake is not automatically actuated within the frameworkof the speed control, but when the driver himself intervenes in theevents and actuates the brake pedal.

However, the fill signal may be switched on once again when, within theframework of the speed control, there is again a switchover from brakingoperation to engine operation, and thus the brake is released. The fillsignal then remains switched on until the acceleration-demand signalagain exceeds the threshold value, or until a predetermined time spanhas elapsed. This measure maintains an increased brake readiness in theevent the acceleration-demand signal constantly fluctuates around thetriggering threshold.

Alternatively, the same thing may also be achieved in that, after therelease of the brake, a special hold signal is generated, the effect ofwhich is that the brake is not released completely, but rather is heldin the preloaded state.

To avoid frequent switching of the control system between engineoperation and braking operation, it may be possible to provide aspecific hysteresis. In this case, the triggering threshold for theswitchover to braking operation is given by an acceleration valuea_(hys) which is less than a_(min), while the switch back to engineoperation takes place when the acceleration-demand signal again becomesgreater than a_(min). If the hysteresis interval between a_(min) anda_(hys) varies dynamically, it is possible to couple the threshold valuefor generating the fill signal to a_(hys). On the other hand, thethreshold value at which the fill signal falls off again after the brakeis released may remain coupled to a_(min).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a speed control system for carrying outa method according to an exemplary embodiment of the present invention.

FIG. 2 shows a timing diagram of signals which occur in the controlsystem according to FIG. 1.

FIG. 3 shows a timing diagram corresponding to FIG. 2 for an alternativeexemplary embodiment of the present invention.

DETAILED DESCRIPTION

In FIG. 1, a motor vehicle is shown symbolically whose speed iscontrolled with the aid of an electronic controller 12. To that end,controller 12 receives a signal from a speed sensor 14 which indicatesthe actual speed of the vehicle. Furthermore, a distance sensor, in theexample shown a radar sensor 16, is mounted in the front on the vehicleand reports distance data and relative-speed data of objects located infront of the vehicle to controller 12. Possibly, radar sensor 16 has acertain angular-resolution capability so that the azimuth angle of thelocated objects may also be detected and reported to controller 12. Inthis manner, the radar system and/or controller 12 are able todifferentiate preceding vehicles in the vehicle's own lane from vehiclesin other lanes, as well as from fixed targets at the edge of theroadway. When preceding vehicles are located in the vehicle's own lane,then the immediately preceding vehicle is selected as a target object,and the speed of motor vehicle 10 is controlled in such a manner that aspecific setpoint distance is maintained to the preceding vehicle. Thissetpoint distance is selectable by the driver by the input of a setpointtime gap which indicates the time interval in which the precedingvehicle and the driver's own vehicle pass the same point on the roadway.Therefore, the setpoint distance is adjusted dynamically to the specifictraveling speed.

If the roadway in front of the driver's own vehicle is clear, then,provided the driver has given a corresponding command, control iscarried out to a desired speed selected by the driver.

Furthermore, controller 12 also evaluates operating commands as well asdriving commands by the driver, particularly the degree of actuation ofthe accelerator and, optionally, of the brake pedal. Consequently, atany time the driver has the possibility of intervening actively inevents in order to react appropriately in critical driving situations.

Therefore, various open-loop and closed-loop control strategies areimplemented in controller 12, and depending on the driving situation orcommands by the driver, one or more control strategies are selected and,as is conventional, their results are combined in a suitable manner toform an acceleration-demand signal a_(setpoint) which indicates thesetpoint acceleration of the vehicle at the moment.

Based on acceleration-demand signal a_(setpoint), a decision unit 18decides whether an intervention in the drive system or in the brakingsystem of the vehicle may be necessary. In response to positive valuesof the acceleration-demand signal, an intervention is carried out in thedrive system. In this case, a control command A is output to anelectronic engine management system 20 which acts on the engine of motorvehicle 10 by way of various actuators, symbolized here by a throttlevalve 22. In general, the functions of engine management system 20 mayinclude the control of throttle valve 22, the control of the fuelinjection system, the ignition and other components of the drive systemof the vehicle. In the case of a vehicle having automatic transmission,this may also include the selection of the transmission step. On thebasis of control command A and the instantaneous operating parameters ofthe engine, engine management system 20 controls the engine so that anengine drive torque is produced corresponding to the acceleration-demandsignal.

When acceleration-demand signal a_(setpoint) assumes negative values,then first of all the engine is throttled by engine management system20, so that the drag torque of the engine is utilized for deceleratingthe vehicle. However, if decision unit 18 determines that the vehicledeceleration thus attainable is not sufficient to keep the actualacceleration of the vehicle in agreement with the setpoint accelerationrepresented by a_(setpoint), then a switchover to braking operation iscarried out by decision unit 18. In this case, the engine remainsthrottled, and decision unit 18 supplies a control command B to a brakecontrol system 24 of the vehicle.

Brake control system 24, via the hydraulic braking system of thevehicle, controls the functioning of brakes 26 allocated to theindividual wheels and fulfills, for example, the function of an antilockbraking system, a traction control system and/or an ESP (ElectronicStability Program) system for dynamic stabilization of the vehicle.

For reasons of failure safety, the hydraulic braking system of thevehicle is coupled directly to the brake pedal of the vehicle andcontains at least one pressure generator or booster which boosts thebraking force exerted by the driver via the brake pedal. Within theframework of the traction control system or the ESP system, the pressuregenerator is also able to build up a braking pressure independently ofthe actuation of the brake pedal and to actuate brakes 26. In the samemanner, control command B transmitted by decision unit 18 also triggersactuation of the brake with a fixed or variable braking force.

If control command B is output when the vehicle brake is not actuatedand the wheel brake cylinders are pressureless, then the dead volumeunavoidably present in the hydraulic braking system and particularly inthe wheel brake cylinders may need to first be filled with brake fluidbefore friction locking between the brake shoes and the brake drums orbrake disks actually occurs and the brake becomes effective. To shortenthe time which may be necessary for filling this dead volume, brakecontrol system 24 has a function which permits a pre-filling of thebraking system. During this process, which shall be designated here as“preloading”, the braking system is pressurized to the extent that thedead volumes are filled up and the brake liners move close to the brakedisks or brake drums or even already lightly contact them. In the lattercase, a slight wear on the brake is accepted.

According to an exemplary embodiment of the present invention, thispreloading function may now not only be triggered within brake controlsystem 24, rather it may also be triggered externally by a suitable fillsignal which is output in the form of a flag F by decision unit 18.Depending on the state of the braking system, the setting of flag Ftherefore results in brake control unit 24 triggering a pre-filling ofthe braking system. If flag F is reset again, then this pre-fillingprocess is canceled by brake control unit 24, provided an actualactivation of the brake has not been implemented by control command B inthe meantime.

Depending on the design of the braking system, it is also possible toimplement in brake control system 24 a “retain” function which providesthat the pre-filled state of the braking system is maintained after thebrake is released. For instance, this may be carried out by temporarilyinterrupting the operation of a return pump which delivers the brakefluid back from the wheel brake cylinder. In this case, the“retain”function may also be initiated by the setting of a correspondingflag H externally by decision unit 18.

The parameters which characterize the state of the engine and thebraking system of the vehicle are available in engine management system20 and brake control system 24, respectively, and may be transmitted toother system components of the vehicle via a data bus 28 (e.g., a CANbus) so that in case of need, they are also available for evaluation incontroller 12 and in decision unit 18.

The changes in the vehicle acceleration caused by the intervention inthe drive system or the brake system of the vehicle lead tocorresponding changes in the vehicle velocity and the distance to thepreceding vehicle, and are coupled back via speed sensor 14 and radarsensor 16.

In FIG. 2, curve 30 in the upper part of the diagram shows an examplefor a time characteristic of acceleration signal a_(setpoint) which isoutput by controller 12. An acceleration value a_(min) represents thesmallest possible (negative) acceleration able to be produced by thedrag torque of the engine under the instantaneous operating conditions.This acceleration value a_(min) represents a triggering threshold atwhich decision unit 18 switches from engine operation to brakingoperation. When a_(setpoint) falls below triggering threshold a_(min) attime t₂, decision unit 18 therefore outputs control command B to brakecontrol system 24.

Triggering threshold a_(min) is first of all a function of operatingparameters of the engine and of the transmission, for example, theengine speed, the instantaneous engine temperature and the like. Thisdata is available via bus system 28 and may be utilized for calculatinga_(min). Moreover, a_(min) is also a function of external conditions,particularly the gradient of the roadway and the inert mass of thevehicle including payload. The corrections to a_(min) caused thereby maybe estimated by comparing acceleration control command A to theresulting actual acceleration of the vehicle. However, in one simplifiedspecific embodiment of the method, a_(min) may also be assumed asconstant. For instance, in one practical example, a_(min) is on theorder of magnitude of −0.5 m/s².

FIG. 2 also indicates a threshold value TH which is greater than a_(min)by a fixed amount Δa. If a_(setpoint) falls below this threshold valueTH at time t₁, then flag F is set and the pre-filling of the brakingsystem is thereby initiated. Time τ needed to pre-fill the brakingsystem is approximately 200 to 300 ms, depending upon the design.Controller 12 is constructed in such a manner that the change rate ofacceleration signal a_(setpoint) over time is limited downwards, forexample, applicable is: d/dt(a_(setpoint)) >—1.0 m/s³. So that thepre-filling of the braking system may be concluded in the time spanbetween t₁ and t₂, the following must therefore be valid: Δa>|τ*d/dt(a_(setpoint))|. Consequently, in the example assumed here,Δa=0.35 m/s² would be a suitable value by which threshold value THshould lie above triggering threshold a_(min). With a_(min)=−0.5 m/s²,it therefore follows that: TH=−0.15 m/s².

When acceleration-demand signal a_(setpoint) falls below triggeringthreshold a_(min) and the brake is actually triggered, then flag F isagain reset. If a_(setpoint) does not reach triggering thresholda_(min), flag F is reset at the latest after a predefined time span ΔThas expired which is determined by signal T of a timer. The timer isstarted when a_(setpoint) reaches threshold value TH (at t₁), and signalT then falls off again after predefined time span ΔT has elapsed. Withthe trailing edge, flag F is also reset again, if it is then stillactive. This case is drawn in with a dotted line in FIG. 2 for thesignal pattern of flag F.

In the example shown, acceleration-demand signal a_(setpoint) risesabove triggering threshold a_(min) again at time t₃, and at this moment,decision unit 18 switches over again to engine operation, so thatcontrol command B output to brake control system 24 falls off again. Atthis moment, flag H is set which causes the braking system to remain inthe pre-loaded state. If the acceleration-demand signal falls belowtriggering threshold a_(min) again without reaching threshold value THin between, the brake may therefore be activated again without delay.Only when acceleration-demand signal a_(setpoint) has risen again abovethreshold value TH (at time T₄), or when time span ΔT has elapsed oncemore, is flag H reset again, and the preloaded state of the brakingsystem is canceled.

In this manner, using a method which may be very simple to implement, itmay be ensured that the braking system of the vehicle reacts with thesmallest possible delay to control command B for activating the brake.

In the simplest case, flags F and H act on the braking system of thevehicle as a whole. However, it is also possible to have these flags actindividually on each single wheel or separately on the front wheels andthe rear wheels. In the event the front wheel brakes and the rear wheelbrakes are constructed differently, Δa may therefore be adjustedindividually to the pre-filling time necessary for the brake inquestion.

FIG. 3 illustrates an exemplary embodiment in which the triggeringthreshold at which control command B is output to brake control system24 is not given directly by a_(min), but rather by a somewhat smalleracceleration value a_(hys). However, the switch back to engine operationtakes place at moment (t₃), at which acceleration-demand signala_(setpoint) again becomes greater than a_(min). This hysteresisfunction prevents decision unit 18 from switching between brakingoperation and engine operation in a “wavering” manner. However,hysteresis interval a_(min)-a_(hys) is not static, but rather is varieddynamically in this example. From the moment at whichacceleration-demand signal a_(setpoint) falls below value a_(min), thehysteresis interval is reduced with constant rate of change to 0, sothat a_(hys) approaches value a_(min). Although a short-duration dropbelow a_(min) may be tolerated, when this undershoot persists longer, byraising triggering threshold a_(hys), a switchover to braking operationmay take place anyway (in the example shown, at time t₂). Thresholdvalue TH, which determines the setting of flag F, is defined here sothat it always lies by a fixed amount Δa above variable triggeringthreshold a_(hys). The flag is reset again upon actual triggering of thebrake or after time span ΔT has elapsed.

Furthermore, FIG. 3 illustrates the case when the function “retain” isnot implemented in brake control system 24, and consequently no flag His set by decision unit 18 either. When the brake is released at timet₃, then in the specific embodiment shown in FIG. 3, flag F is set oncemore so that the increased reaction readiness of the brake is stillretained for a certain time. When acceleration-demand signala_(setpoint) rises further and exceeds a threshold value TH′, which isgreater by Δa than a_(min), flag F is reset again (at time t₄). However,the flag may be reset after time span ΔT has elapsed.

On its part, time span ΔT set in the timer may be varied in bothexemplary embodiments as a function of the operating state.

1. A method for controlling a speed of a vehicle, comprising: generatingan acceleration-demand signal which represents one of a positivesetpoint acceleration of the vehicle and a negative setpointacceleration of the vehicle; outputting one of a first control commandto an engine of the vehicle and a second control command to a brakingsystem of the vehicle as a function of the acceleration-demand signal;outputting a signal for preloading the braking system when theacceleration-demand signal falls below a threshold value which is abovea value at which the braking system is activated; determining atriggering threshold as a function of a vehicle deceleration; andrestricting a change rate of the acceleration-demand signal over time bya lower limit.
 2. The method as recited in claim 1, wherein: thetriggering threshold is that at which the braking system is activated bythe output of the second control command, and the vehicle decelerationis that which is produceable by a drag torque of the engine underinstantaneous operating conditions; and the threshold value, at whichthe signal for preloading the braking system is output, is above thetriggering threshold by a specific amount.
 3. The method as recited inclaim 1, further comprising: resetting the signal for preloading thebraking system when the braking system is actually activated.
 4. Themethod as recited in claim 3, further comprising: when the brakingsystem is deactivated, one of renewedly setting the signal forpreloading the braking system, and setting a signal for retaining thepreloading of the braking system.
 5. The method as recited in claim 4,further comprising: resetting one of the renewedly set signal forpreloading the braking system and the signal for retaining thepreloading when the acceleration-demand signal exceeds the thresholdvalue.
 6. A method for controlling a speed of a vehicle, comprising:generating an acceleration-demand signal which represents one of apositive setpoint acceleration of the vehicle and a negative setpointacceleration of the vehicle; outputting one of a first control commandto an engine of the vehicle and a second control command to a brakingsystem of the vehicle as a function of the acceleration-demand signal;outputting a signal for preloading the braking system when theacceleration-demand signal falls below a threshold value which is abovea value at which the braking system is activated; and determining atriggering threshold, at which the braking system is activated by theoutput of the second control command, as a function of a vehicledeceleration that is produceable by a drag torque of the engine underinstantaneous operating conditions, wherein the threshold value, atwhich the signal for preloading the braking system is output, is abovethe triggering threshold by a specific amount; restricting a change rateof the acceleration-demand signal over time by a lower limit, whereinthe specific amount is greater by an amount than a product of the lowerlimit and a time necessary for preloading the braking system.
 7. Themethod as recited in claims 6, further comprising resetting the signalfor preloading the braking system when the braking system is actuallyactivated.
 8. The method as recited in claim 7, further comprising: oneof, when the braking system is deactivated, renewedly setting the signalfor preloading the braking system, and setting a signal for retainingthe preloading of the braking system.
 9. The method as recited in claim8, further comprising: resetting one of the renewedly set signal forpreloading the braking system and the signal for retaining thepreloading when the acceleration-demand signal exceeds the thresholdvalue.
 10. The method as recited in claim 8, further comprising:resetting at least one of the signal for preloading the braking systemand the signal for retaining the preloading after a time span haselapsed which is one of fixed and variable as a function of theinstantaneous operating conditions.
 11. A method for controlling a speedof a vehicle, comprising: generating an acceleration-demand signal whichrepresents one of a positive setpoint acceleration of the vehicle and anegative setpoint acceleration of the vehicle; outputting one of a firstcontrol command to an engine of the vehicle and a second control commandto a braking system of the vehicle as a function of theacceleration-demand signal; outputting a signal for preloading thebraking system when the acceleration-demand signal falls below athreshold value which is above a value at which the braking system isactivated; resetting the signal for reloading the braking system whenthe braking system is actually activated; when the braking system isdeactivated, one of renewedly setting the signal for preloading thebraking system, and setting a signal for retaining the preloading of thebraking system; and resetting at least one of the signal for preloadingthe braking system and the signal for retaining the preloading after atime span has elapsed which is one of fixed and variable as a functionof the instantaneous operating conditions.
 12. The method as recited inclaim 11, further comprising: determining a triggering threshold as afunction of a vehicle deceleration; and restricting a change rate of theacceleration-demand signal over time by a lower limit.
 13. The method asrecited in claim 12, wherein: the triggering threshold is that at whichthe braking system is activated by the output of the second controlcommand, and the vehicle deceleration is that which is produceable by adrag torque of the engine under instantaneous operating conditions; andthe threshold value, at which the signal for preloading the brakingsystem is output, is above the triggering threshold by a specificamount.
 14. The method as recited in claim 13, wherein the specificamount is greater by an amount than a product of the lower limit and atime necessary for preloading the braking system.